Many polymeric materials, i.e. materials consisting of polymers, must have fire-resistant properties for their end use.
In a fire situation, the aim is mainly to prevent the fire from spreading because of the presence of fabrics, tarpaulins and wall coverings based on polymeric materials and/or to limit physical deterioration of the polymer when it is used, for example in a protective garment, or even ultimately to trap the toxic volatiles resulting from the decomposition of the polymeric materials.
Hitherto, organic or inorganic additives were added when producing compositions of polymeric materials, these being called “compounds” when the polymeric material is extruded or “formulations” when the polymeric material is deposited by coating.
The organic additives used as fire retardants are often, at the present time, molecules containing bromine or fluorine. Their use is therefore limited to certain usages because of the toxicity of the combustion by-products. One combustion by-product that is particularly hazardous is hydrofluoric acid, HF.
The inorganic additives used hitherto are hydroxides or oxyhydroxides of transition metals, or of metals of groups III and IV of the Periodic Table of the Elements, or else carbonates of these transition metals or metals of groups III and IV. However, inorganic additives of the latter type decompose, releasing CO2.
The use of inorganic additives of the hydroxide type is based principally on the oxolation mechanism: 2M-OH→MO+H2O.
Under the action of heat, the hydroxyl groups on the surface of the inorganic particles condense, releasing water, and this water, on vaporizing, will “cool” the system, thereby slowing down or even stopping flame spread.
There is therefore a direct relationship between the number of available hydroxyl groups and the potential of the inorganic particle as fire-resistant filler. Likewise, it is preferable to use mineral fillers of nanoscale size, these developing very high specific surface areas and thus increasing the number of available hydroxyl groups on the surface.
The formulator must therefore “fill” its polymeric material in such a way that the composite obtained has the desired usage properties, in particular in terms of surface appearance, mechanical strength, etc. and also in terms of fire resistance corresponding to the standards of the field of use.
A compromise must be found in respect of the inorganic filler/polymer ratio.
Thus, for example, in the case of polyvinyl chloride (PVC) materials, the flame retardants are antimony-hydroxides or tin hydroxides.
The formulator must also take into account chemical incompatibilities that preclude mixing just any filler with just any polymer. Moreover, the fact that in crystalline nanofillers the energy of the surface hydroxyl groups is not at all homogeneous, for certain surface hydroxyl groups the oxolation will take place only at very high temperature, thereby limiting the advantage thereof for many usages, the damage having already taken place.
The formulator must also take into account future recycling of the fire retardant/polymer composite and eliminate environmentally toxic substances. Now, the antimony and tin compounds currently used are toxic substances.
There is therefore a need for a fire retardant that does not produce toxic by-products, has a very large number of surface hydroxyl groups that can undergo oxolation at low temperature, is pure and can be modified so as to be compatible with the largest possible number of polymeric matrices.